1. Field of the Invention
The present invention relates to a fixing device having a planar member, which is relatively pressed against a rotary member being rotated and includes a heat conduction layer having a predetermined thickness and causing a recording sheet that bears an unfixed toner image to pass between the planar member and the rotary member to thereby apply heat and pressure to the unfixed toner image so that the unfixed toner image is fixed onto the recording sheet, to a sheet member disposed to the fixing device, and to an image forming apparatus including the fixing device.
2. Description of the Related Art
Among image forming apparatuses such as a copy machine, a printer, a facsimile, and the like that employ an electrophotographic system, there is known an image forming apparatus including a fixing device that fixes an unfixed toner image transferred onto a recording sheet such as a paper sheet and the like onto the recording sheet. As a type of the fixing device, there is known a fixing device that forms a nip region between the peripheral surfaces of a pair of rotatable rotary members which are caused to come into contact with each other, causes a recording sheet that bears an unfixed toner image to pass through the nip region to thereby apply heat and pressure to the unfixed toner image so that it is fixed onto the recording sheet. There is known, for example, a so-called roll-roll type fixing device using a heat roll having a heat source disposed therein and a pressure roll having an elastic layer formed therearound as a pair of rotatable rotary members. However, in the roll-roll type fixing device, the heat roll is also provided with a somewhat thick elastic layer formed therearound to form a nip region longer than a predetermined length. A problem arises from the above arrangement in that the heat capacity of the heat roll is increased, and thus a time (herein after, referred to as “warm up time”) necessary to heat the heat roll from a room temperature to a fixing possible temperature is increased. To cope with the above problem, there is proposed a so-called roll-belt type fixing device using an endless belt in place of the pressure roll (for example, Japanese Patent Application Laid-Open No. 9-34291).
FIG. 1 is a schematic arrangement view of an example of a conventional roll-belt type fixing device.
The fixing device 300 shown in FIG. 1 has a main portion including a heat roll 310 having a heat source contained therein, an endless fixing belt 320 stretched around three support rolls 321, 322, and 323, and a pressure application member 330 that is abutted against the inner peripheral surface of the fixing belt 320 and presses the fixing belt 320 along the surface of the heat roll 310.
The heat roll 310 has a cylindrical core 311 disposed therein and is driven in rotation by a not shown motor in the direction of an arrow in the figure. The core 311 has an elastic layer 312 formed on the surface thereof, and the surface of the elastic layer 312 is covered with a mold release layer 313. That is, the surface of the heat roll 310 is formed of the mold release layer 313. The core 311 has a halogen lamp 314 as a heat source disposed therein. Further a mold releaser application unit 340 is disposed in the vicinity of the heat roll 310 to apply mold release oil as a mold releaser. In the mold releaser application unit 340, the mold releaser 342 dropped from an oil pipe 341 is supplied to a pick-up roll 344 through an oil wick 343, and the mold releaser 342 excessively supplied to the pick-up roll 344 is scraped off by a metering blade 345 and returned from an oil pan 346 to a not shown oil tank. Further, the mold releaser 342 supplied to the surface of the pick-up roll 344 is applied onto the surface of the heat roll 310 through a donor roll 347. Note that the materials deposited on the surface of the pick-up roll 344 are eliminated with a cleaning blade 348. Further, an external heat roll 350 is disposed on the surface of the heat roll 310 to heat the surface of the heat roll 310 at a predetermined timing in contact with the surface. Further, a temperature sensor 380 is disposed on the surface of the heat roll 310 to control the surface temperature of the heat roll 310.
The fixing belt 320 is circulated by the rotation of the heat roll 310 with the surface thereof pressed against the heat roll 310. One of the three support rolls 321, 322, and 323, that is, the support roll 321, around which the fixing belt 320 is stretched, has a heater lamp 3211 disposed therein.
The pressure application member 330 is composed of a metal base plate 331 and an elastic layer 334 disposed on the surface of the base plate 331, and the elastic layer 334 is composed of a silicon rubber foamed member and laminated on a metal support plate 333 through a shim 332. Further, the entire peripheral surface of the pressure application member 330 is covered with a low friction sheet 335 as a sheet-shaped member. The low friction sheet 335 is provided to reduce the sliding resistance between the pressure application member 330 and the fixing belt 320. The low friction sheet 335 is composed of a material having a heat resistant property and a wear resistant property and has large concavo/convex portions formed on the surface thereof. The pressure application member 330 is urged to the heat roll 310 by a not shown compression coil spring disposed to the base plate 331 side with press force of, for example, 50 kgf. The contact surface of the low friction sheet 335 in contact with the fixing belt 320 can be aligned with the surface of the heat roll 310 by disposing the elastic layer 334 to the pressure application member 330. That is, when the pressure application member 330 is pressed against the heat roll 310 with a load of a predetermined magnitude or more, the elastic layer 334 is deformed and the contact surface of the low friction sheet 335 is deformed along the outer peripheral surface of the heat roll 310. Accordingly, when the pressure application member 330 is pressed against the heat roll 310 by the not shown compression coil spring, the outer peripheral surface of the fixing belt 320 is caused to come into pressure contact with the surface of the heat roll 310 without intervals there between while the inner peripheral surface of the fixing belt 320 is supported by the pressure application member 330. The outer peripheral surface of the fixing belt 320 is pressed against the heat roll 310 by the pressure application member 330 and the support roll 323 disposed in the vicinity of the pressure application member 330, thereby a nip region N is formed.
Silicon oil is applied to the inner peripheral surface of the fixing belt 320 by a lubricant application member 360 composed of felt and the like, thereby the sliding resistance between the fixing belt 320 and the low friction sheet 335 can be reduced. When the silicon oil is applied, the fixing belt 320 can be caused to travel by the rotation of the heat roll 310 while sliding on the low friction sheet 335 at a speed approximately the same as the rotational speed of the heat roll 310.
In FIG. 1, a toner image T is transferred onto a sheet P by a not shown transfer unit on the left side of the figure, and the sheet P that bears the unfixed toner image T is transported to the nip region N of the fixing device 300 shown in FIG. 1. When the sheet P enters the nip region N and passes therethrough, the unfixed toner image T is heated and pressed, thereby the unfixed toner image T is fixed onto the sheet P.
In the roll-belt type fixing device 300 shown in FIG. 1, since the portion of the fixing belt 320 in pressure contact with the peripheral surface of the heat roll 310 is formed in the shape along the peripheral surface of the heat roll 310, the nip region N longer than a predetermined length is formed more easier than in the roll-roll type fixing device. Accordingly, the heat roll 310 shown in FIG. 1 is formed thinner than that of the heat roll disposed to the roll-roll type fixing device, thereby the warm-up time can be reduced.
Incidentally, sheets having plural sizes are fed to the nip region N of the fixing device. As shown in FIG. 1, in the fixing device, in which the thickness of the elastic layer 312 of the heat roll 310 is made thin to reduce the heat capacity of the heat roll 310, when a sheet having a width smaller than a maximum sheet passing width is passed, that is, when an A4 sheet is longitudinally passed through a fixing device, through which an A3 sheet, for example, can be passed, or when a B4 sheet is passed therethrough, both the end portions of the nip region N in the width direction thereof (direction perpendicular to a sheet surface in FIG. 1) are made to regions through which no sheet is passed (herein after, referred to as “non-sheet-pass regions). Since the heat of the non-sheet-pass regions is not absorbed by a sheet, the temperature of the non-sheet-pass regions is significantly increased. When the temperature of the non-sheet-pass regions is significantly increased, a problem arises in that the fixing belt is deteriorated by heat or peripheral components are damaged. Further, the low friction sheet 335 shown in FIG. 1 is deteriorated by heat, and the silicon oil applied to the front surface of the low friction sheet 335 is also deteriorated, thereby the sliding resistance of the fixing belt is increased, the load torque of the fixing device is increased, and sheet get wrinkled and images are offset by a sheet transport failure. In addition to the above drawbacks, nip pressure is distributed in a state different from an ordinary state due to the uneven amount of thermal expansion of the pressure application member 330, from which a problem arises in that sheets get wrinkled and are curled, and images are unevenly fixed on sheets. Further, when an A3 sheet is passed in the state that the temperature of the non-sheet-pass regions is significantly increased, the A3 sheet comes into contact with the non-sheet-pass regions whose temperature is significantly increased and is excessively heated, thereby toner is hot offset or the amount of curl of the sheet is increased.
Conventionally, various approaches are contemplated to cope with these problems, and some of them can be applied to the roll-belt type fixing device as shown in FIG. 1.
For example, as one of the approaches, it is contemplated to dispose plural heat sources, which are classified according to sheet sizes, in the heat roll to heat the nip region and to switch power supplied to the heat sources according to the size of a sheet to be passed.
In this case, however, problems arise in that it is difficult to dispose the plural heat sources in the heat roll because the diameter of the heat roll is recently reduced to satisfy the requirement for the reduction in size of the fixing device, that a cost is increased by the increase of the number of the heat sources, and that a control becomes complex because the plural heat sources must be switched. In view of the these problems, it is difficult to dispose heat sources corresponding to all the sheet sizes, and actually, two or three heat sources are disposed, which is insufficient to overcome the outstanding temperature increase in the non-sheet-pass regions.
Further, as other approaches, although it is also contemplated to suppress the increase of temperature of the non-sheet-pass regions by relatively separating the heat roll from the fixing belt or reducing the passing speed of sheets, these approaches reduce the number of sheets that can be fixed per unit time, and the speed-up of the fixing device is prevented.
Further, there have been proposed approaches for solving these problems mechanically in the conventional roll-roll type fixing device (refer to, for example, Japanese Patent Application Laid-Open Nos. 8-87191, 2004-53674, and 8-286555. Japanese Patent Application Laid-Open No. 8-87191 proposes a technique for reducing the outstanding temperature increase of non-sheet-pass regions by causing a high heat conductive member to come into contact with the surface of a heat roll from the outside. However, this proposal is not preferable because the surface of the heat roll is likely to be scratched by the contact thereof with the high heat conductive member. To cope with the above problem, Japanese Patent Application Laid-Open No. 2004-53674 proposes to dispose a high heat conductive member so that it is free to come into contact with and to separate from the surface of a fixing member. In this proposal, however, since a mechanism is additionally necessary to dispose the high heat conductive member so that it is free to come into contact and to separate from the fixing member, thereby the cost and the size of a fixing device are increased. Although Japanese Patent Application Laid-Open No. 8-286555 proposes a technique for reducing the increase of temperature of non-sheet-pass regions by disposing a heat pipe in the metal core of a pressure roll disposed in confrontation with a heat roll, a cost is increased by disposing the heat pipe. Further, since the diameter and the wall thickness of recent pressure rolls are reduced, it is difficult to assemble the heat pipe to the thick portion of the core from a view point of space. Furthermore, in the fixing device as shown in FIG. 1 in which the pressure roll is replaced with the fixing belt, it is primarily impossible to dispose the heat pipe and the like to the fixing belt. Accordingly, when the heat pipe is to be disposed, it must be disposed in a heat roll. In this arrangement, however, the heat roll cannot sufficiently exhibit its function, and the problems cannot be solved.